Of the sunlight reaching the earth (including infrared light, visible light, and ultraviolet light), 5 to 6% is ultraviolet light. The ultraviolet light has short wavelengths, and thus comprises high-energy electromagnetic waves. Therefore, the ultraviolet light has been widely known to have degradability against many kinds of substances, thereby causing damages to living bodies.
Therefore, ultraviolet shielding agents have been used for applications of protecting skin from causing inflammation or skin cancer due to the ultraviolet light by formulating them in cosmetics, or preventing a pigment from discoloration due to decomposition by ultraviolet light by mixing them in paints. By such applications, an unnatural whitening in cases of the cosmetics or a loss of coloration in cases of the paints can be prevented by increasing the transparency in the visible light region. Therefore, it is desired that the ultraviolet light is protected while the transparency in the visible light region is maintained.
The ultraviolet shielding agent comprising organic compounds as effective ingredients prevents the transmission of the ultraviolet light on account of the specific absorption of the composition against the ultraviolet light. For example, an ultraviolet absorbable composition comprising substituted N,N'-bis-aromatic formamidines, and the like are known (Japanese Patent Examined Publication No. 61-09993). However, while absorbing the ultraviolet light, the organic ultraviolet shielding agents have the problem of degradation on accounts of the absorption, with the result of an undesirable lowering of the shielding abilities with the passage of time. In their applications to cosmetics, the kinds and amounts of the ultraviolet shielding agents formulated are restricted owing to effects caused on human bodies, and thus it is difficult to achieve a good shielding performance within a controlled range.
On the other hand, in the ultraviolet shielding agent using an inorganic compound, the composition is formulated with inorganic fine particles, and the transmission of the ultraviolet light is prevented by the absorbing ability and the scattering ability against the ultraviolet light owned by the composition. In the inorganic ultraviolet shielding agent, the composition is not degraded with the passage of time and causes little damages on the human bodies, so that it is superior to the organic ultraviolet shielding agent. However, when compared with organic ultraviolet shielding agents, since the inorganic ultraviolet shielding agents are in the form of particles, it has been conventionally considered to be rather difficult to protect against the ultraviolet light with inorganic ultraviolet shielding agents while maintaining high transparency in the visible light region.
In order to effectively exhibit the shielding abilities in the ultraviolet light region while maintaining high transparency in the visible light region (light wavelengths of from 400 to 800 nm), the composition has to be made ultrafine to be in a state of high dispersion to thereby increase the shielding abilities against the ultraviolet light. However, in the case of using such ultrafine particles, problems may arise in the dispersion stability due to the aggregation ability of the ultrafine particles, and in the catalytic activities of the ultrafine particles.
In order to improve dispersibility, there have been known methods of improving surfaces of the ultrafine particles by coating with other substances, including, for instance, an invention concerned with cosmetics formulating powdery products of zinc oxide fine particles coated with an anionic surfactant (Japanese Patent Examined Publication No. 5-77644); an invention concerned with a method of producing powdery products of zinc oxide ultrafine particles coated with an anionic surfactant (Japanese Patent Laid-Open No. 62-260716), and the like. In the above publications, the method comprises the steps of surface-treating the particles in an aqueous solution, solvent-replacing with an organic solvent, and drying, in which the number of the production steps is large, and also process is complicated. Also, in publications other than those mentioned above, there have been known, for instance, an invention concerned with a composition for cosmetics comprising metal oxide fine particles coated with a surfactant and a method of producing the same (Japanese Patent Laid-Open No. 62-84017). In this publication, the method comprises the steps of surface-treating the particles in a liquid phase, aggregating and filtering, washing, dehydrating, and blending, in which the number of the production steps is extremely large as in the method mentioned above, and the process is very complicated.
In publications other than those mentioned above, there have been known, for instance, an invention concerned with a method of producing zinc oxide fine particles obtainable by heating a mixture comprising zinc or a zinc compound, a carboxyl group-containing compound, and an alcohol (Japanese Patent Laid-Open No. 7-232919); an invention concerned with composite oxides for shielding against ultraviolet light obtainable by the steps of reacting an acidic solution containing at least one member selected from zinc, lanthanum, cerium are the like with an alkali solution, filtering, washing, drying, firing, and subsequently surface-treating with a silicone oil or a fatty acid, and a method of producing the same (Japanese Patent Laid-Open No. 5-222317), and the like. In these publications, the reaction step is an essential step in the production processes, which is noted to accompanied the complication of controlling the reaction.
In publications other than those mentioned above, there have been known an invention concerned with cosmetics characterized by containing a powder obtainable by subjecting titanium oxide to a coating treatment with mixed hydrates comprising particular amounts of silicate hydrates and alumina hydrates, wherein the titanium oxide is nearly spherical or irregularly shaped and has an average particle size of from 30 to 70 nm, and further subjecting the surfaces to a coating treatment with a silicone oil (Japanese Patent Laid-Open No. 2-247109). However, the process of coating treatment with mixed hydrates comprising the silicate hydrates and the alumina hydrates before the particle surface is subjected to a surface treatment with a silicone oil is required, which is noted to be accompanied with the complication in the reaction control of the mixed hydrates.
Next, for the purpose of providing a material dispersed with fine particles as to be used as water-repellant, ultraviolet shielding agents for sunscreen cosmetics, there have been known, for instance, an invention concerned with an oily, dispersed material obtainable by the step of pulverizing a mixture containing an oil, titanium dioxide particles, and an organic dispersant in the presence of a granular pulverizing medium, and a method of producing the same (Japanese Patent Examined Publication No. 6-61457); an invention concerned with a method of producing sunscreen obtainable by the step of pulverizing a mixture containing an oil, zinc oxide, titanium dioxide, and an organic dispersant in the presence of a granular pulverizing medium (Japanese Patent Laid-Open No. 5-201844); an invention concerned with a material dispersed with zinc oxide fine particles obtainable by the step of pulverizing a mixture containing an oil, zinc oxide particles, and an organic dispersant in the presence of a granular pulverizing medium (Japanese Patent Laid-Open No. 5-213618), and the like. In these publications, since the particle concentration in the dispersed material is as high as not less than 30%, its production is difficult, and the stability of the dispersed material is low. In addition, a silicone oil which is suitable to be formulated in sunscreen cosmetics as a dispersion medium from the aspects of good skin texture and good stability when used as cosmetics is not used, nor a dispersant suitable for the silicone oil is used. Therefore, there is a limitation of the amount of these dispersed materials formulated in sunscreen cosmetics, so that the dispersibility of the particles and the stability of the dispersed material are poor.
Also, there have been known an invention concerned with a dispersed liquid of titanium dioxide obtainable by the step of pulverizing a mixture containing titanium dioxide and an organic compound having a suitable branched chain in the absence of a dispersing aid, a cosmetic composition thereof, and a method of using the same (Japanese Patent Unexamined Publication No. 8-507081). In this publication, since an organic compound having a branched chain acting as a dispersant is used as a dispersion medium, the amount formulated in cosmetics is limited.
In publications other than those mentioned above, there have been known, for instance, an invention concerned with cosmetics formulated with inorganic powdery products of fine particles surface-treated with an organic silicon compound by a wet pulverization or a wet disintegration using a medium agitation mill (Japanese Patent Laid-Open No. 8-104606); an invention concerned with an oily, dispersed material comprising titanium oxide fine particles surface-improved with a particular alkylalkoxysilane dispersed in a water-repellent solvent or oily agent (Japanese Patent Laid-Open No. 8-119832); an invention concerned with cosmetics comprising metal oxide ultrafine particles having a particle size of not more than 0.1 .mu.m, a dispersion medium, and a dispersant, the ultrafine particles having a particle size of the dispersed particles of not more than 0.1 .mu.m, the content of the fine particles being not less than 10% by weight (Japanese Patent Laid-Open No. 6-239728), and the like. In these publications, since siloxanes and silanes are used as dispersants, the firing step is required.
Further, in publications other than those mentioned above, there has been known an invention concerned with a colloidal zinc oxide obtainable by the steps of heating zinc carbonate to form zinc oxide agglomerates, adding the agglomerates in a polyacrylic acid dispersed liquid, and pulverizing them (Japanese Patent Unexamined Publication No. 8-510440). In this publication, the method requires a reaction process, which would be noted to have complications in reaction control.
Also, for the purpose of providing a material dispersed with aqueous ultrafine particles, the ultrafine particles being stably dispersed to be used as hydrophilic ultraviolet shielding agents used in such applications as sunscreen cosmetics, there have been proposed, for instance, an invention concerned with a dispersed material obtainable by pulverizing a mixture of water, acicular titanium dioxide and a polycarboxylic acid or a salt thereof used as a dispersant in the presence of a granular pulverizing medium (Japanese Patent Laid-Open No. 2-212315); an invention concerned with an aqueous dispersed material containing water as a dispersion medium, titanium oxide ultrafine particles and a nonionic surfactant, the material dispersed with titanium dioxide of which particle surfaces of the titanium oxide ultrafine particles are subjected to a hydrophobic treatment (Japanese Patent Laid-Open No. 7-247119), and the like. The material dispersed with the fine particles in the above publication is an aqueous dispersed material.
In order to solve the various problems in the ultraviolet shielding agents using the metal oxide fine particles mentioned above, in an invention concerned with ultraviolet shielding composite fine particles, a method for producing the same, and cosmetics (International Unexamined Publication No. WO95/09895, Japanese Patent Laid-Open No. 8-12961), an ultraviolet shielding agent having high transparency in the visible light and high shielding abilities against the ultraviolet light is developed by forming composites comprising the fine particles having shielding abilities (daughter particles) against the ultraviolet light and aggregates of fine particles (matrix particles) in which the daughter particles are dispersed, and by determining the combination thereof based on the heights of the band gap energies of the daughter particles and the matrix particles, so that it is made possible to optimize the optical properties owned by the ultrafine particles. This ultraviolet shielding agent is characterized by the following. The refractive index can be controlled in a wide range by changing the materials and proportions of the matrix/daughter particles, so that high transparency can be exhibited regardless of the shapes, the high transparency being exhibited even when dispersed in various media. The handleablity (transportation, surface treatment, blending, and the like) is made easy owing to the sizes of the order of the fine particles. The coloration is not impaired, thereby making it usable for cosmetics. However, when using this ultraviolet shielding agent for cosmetics, the amount of the ultraviolet shielding agent formulated has to be made large in cases where the shielding abilities against the ultraviolet light are to be made large. In such cases, the texture of the powdery product of the composite fine particles becomes too stiff, so that an upper limit of the formulated amount must be set so as not to impair the texture of the cosmetics. Also, when used for cosmetics, it is necessary to inhibit the catalytic activities owned by the ultrafine particles located near the surfaces of the composite fine particles. Also in publications other than those mentioned above, there have been known an invention concerned with solid, porous silica beads containing metal compounds, a method of producing the same, and powdery deodorant (Japanese Patent Laid-Open No. 4-65312); an invention concerned with zinc oxide-polymer composite fine particles, a method for producing the same, and use therefor (Japanese Patent Laid-Open No. 8-60022); an invention concerned with zinc oxide fine particles formed by composites comprising crystalline co-precipitates of zinc and a particular metal element added thereto, and use therefor (Japanese Patent Laid-Open No. 8-253317), and the like. In the above publications, as in the above cases, by using these composite fine particles for cosmetics, the amount of the composite fine particles formulated has to be made large in cases where the shielding abilities against the ultraviolet light have to be made large. In such cases, the texture of the powdery product of the composite fine particles becomes too stiff, so that an upper limit of the formulated amount must be set so as not to impair the texture of the cosmetics. Also, when used for cosmetics, it is necessary to inhibit the catalytic activities owned by the ultrafine particles located near the surfaces of the composite fine particles.
Therefore, in order to solve the problem of posing limitations in the shielding abilities against the ultraviolet light owing to the upper limit in the formulation mentioned above and the problem of inhibiting the catalytic activities of the composite fine particles, in an invention concerned with ultraviolet shielding composite fine particles, method for producing the same, and cosmetics (Japanese Patent Laid-Open No. 9-100112), further adding the features of remarkably increasing the degree of freedom of formulation by making an average particle size of the composite fine particles small and reducing the powdery texture to thereby increase the upper limit of formulation, and of substantially inhibiting the catalytic activities of the composite fine particles by coating the surface of the composite fine particles with an inorganic substance having substantially no catalytic activities to the designed concept of the ultraviolet shielding composite fine particles mentioned above (International Unexamined Publication No. WO95/09895 and Japanese Patent Laid-Open No. 8-12961), to solve the above problems. However, in the publication (Japanese Patent Laid-Open No. 9-100112), in the method for producing the ultraviolet shielding fine particles, after the particles are surface-treated, the steps of solvent-replacing and dispersing in the oily agent is required, which involves a large number of production steps, thereby making the method complicated.